This invention relates to epoxy resin compositions, and more particularly to epoxy resin compositions suitable for use as encapsulating materials for semiconductor devices and the like.
Generally, epoxy resins are cured using hardeners, such as amines, acid anhydrides, phenol resins, etc. to yield products having excellent electrical, mechanical and thermal properties. These properties make the epoxy resins suitable for use as encapsulating materials for protecting semiconductor devices and other electronic components from the environment or mechanical shocks. Plastic encapsulation using epoxy resins also presents productivity and economic advantages compared to encapsulating with ceramics or metals. For these reasons plastic encapsulation is used more frequently.
Recent advances in the art have resulted in semiconductor devices being more highly integrated. Accordingly, such highly integrated devices are being used in a greater variety of situations than heretofore. As the uses of such devices have increased, the need for reliability and maintenance of the electrical performance at high temperatures and high humidity has also increased. However, conventional epoxy resins possess fundamental problems under such conditions as will be described below. Accordingly, it has been difficult to provide an epoxy resin encapsulated device having the desired electrical characteristics at high temperature and high humidity.
Devices encapsulated with conventional epoxy resin compositions have low moisture resistance and corrosion resistance. This is attributable to the fact that the resin and the electrical device are in direct contact with each other and that the plastic encapsulation is not hermetic. Improvements have been extremely difficult. The epoxy resins absorb moisture and are permeable to moisture due to polar groups remaining in the cured products. Additionally, epoxy resins contain ionic impurities, such as chlorine derived from the epichlorohydrin used as a starting material, sodium derived from sodium hydroxide used for dechlorination, etc.
The ionic impurities are present in high concentration in the raw materials. Thus, interaction between the impurities and the moisture due to absorption or permeation results in a reduction in electrical performance. There is a reduction in the insulating properties of the plastic encapsulating the electrical parts which results in an increase in leakage current and the like. This may even cause corrosion of aluminum wiring and electrodes used in the devices which result in failure of the device.
At high temperatures the ionic impurities and other polar substances present in the resins become more mobile with an increase in activity due to thermal motion. When an electric field is generated in a portion of a device that ionic impurities become more active at the interface of the resin and the element. This lowers the electrical characteristics locally. If water is present corrosion rapidly proceeds and exerts an adverse effect.
In order to overcome these problems and maintain the electrical characteristics of a resin encapsulated device at high temperature and high humidity, various proposals have been made. For example, one such suggestion is to lower the reduction in the volume resistivity of the epoxy resin composition by the addition of an organosilicon compound. Such organosilicon compounds include, alkylarylsilsesquioxane silicon compounds having the chemical formula A described in Japanese Patent Application, Laid Open No. 81333/1981, as follows: ##STR1## wherein R.sup.1 to R.sup.6, inclusive, are the same or different and each represents alkyl, aryl, alkenyl or aralkyl. Other organosilicon compounds suggested as additives include organosiloxane polymers having the general formula B, described in Japanese Patent Publication Nos. 29720/1978 and 17536/1983, as follows: ##STR2## wherein R.sup.7 -R.sup.16, inclusive, are the same or different and each represents alkyl, aryl, alkenyl, aralkyl, hydrogen or hydroxy, and n and m are independently 0 or an integer of 1 or more.
The electrical insulating property of resins including these additives are good at high temperatures and high humidity. However, the moisture resistance of resins including these organosilicon compound additives is not fully satisfactory. This is believed due to the ionic impurities in the epoxy resins.
There are other proposals for improving the moldability when encapsulating with the plastics and for increasing the compatibility between the resin and the elements of the device by adding an organotin compound. Such organotin compounds include di-n-butyltin dimalate, dioctyltin dilaurate, etc. as described in Japanese Patent Publication No. 33470/1978 and Japanese Patent Application Laid Open No. 126842/1982. However, even when including the tin additives, the moisture resistance and the adverse effect due to the ionic impurities in the epoxy resin require further improvement.
Recently, with the advances in high integration, semiconductor devices have become larger in size and the aluminum wiring included therein has become more minute. Additionally, the packages including the devices are becoming thinner and are often presented in articles having flat configurations. Under these circumstances, there is a need for an encapsulating plastic having high moisture resistance coupled with reduced stresses. Further, since high integration increases heat generation, extremely strenuous requirements which do not permit reduction in heat resistance have been made.
In order to overcome these additional problems, various proposals are suggested in the art. One example includes forming a low stress epoxy resin composition, wherein internal stress due to contraction caused by curing when encapsulating an electrical device is reduced by adding a so-called telechelic polymer. One such example is a liquid rubber having a structure wherein functional groups are present at both ends of the molecule. These are described in Japanese Patent Application Laid Open Nos. 180626/1982 and 131223/1982. While these additives reduce stress in the epoxy resin compositions, the moisture resistance is poor. This causes an adverse influence due to the presence of the ionic impurities as is the case with the organosilicon compound additives discussed above.
Finally, it has also been proposed to increase moisture resistance by reducing the ionic impurities present in the epoxy resin, or increasing the adhesion between the electrical elements and the resin. However, it remains difficult to remove the ionic impurities from the resin completely. This presents a problem of mold release and adhesion between the electrical part and the resin increases.
Accordingly, it is desirable to provide an epoxy resin composition particularly well suited for use as an encapsulating material for electrical elements, such as semiconductor devices which overcomes the problems associated with prior art compositions.